Really depends on your set up...
Given that you have a 250W HPS and 80x80x160cm tent
I would say 8L or 11L at most..
You don't want too big of a pot unless you actually have the space to let it grow.
If you go with bigger pots, you'll have bigger plants and have to deal with fitting them in the tent and end up getting light burn from growing too tall.
If she's the only girl in the tent then go ahead for the 20L, do some LST and she'll fit nicely but taking up ALL that space EASILY.
Don't think only on the POSSIBLE harvest, also think about the POSSIBLE problems that could ruin your whole grow cycle..
Good luck and Happy Growing!😃🌱🌿🌲